The present invention relates generally to a method and apparatus for rotationally applying polymers and more particularly to a novel method and apparatus for lining structures such as tanks and the like. The present invention also relates to an improved apparatus for molding hollow plastic articles and to an improved tank or vessel with a protective polymeric lining.
Rotational molding is a known process utilized to form hollow plastic vessels and the like. Generally, in a known rotational molding process, a polymeric material such as polyethylene is loaded into a mold and the mold is subjected to heat such as in an oven while it is simultaneously rotated about two axes. As the mold is rotated, and the polymeric material melts, the polymeric coats the interior of the mold to form a hollow structure with a substantially uniform wall thickness. After an appropriate amount of time, the mold is removed from the oven and cooled so as to solidify the polymeric material. Then, the mold is opened and the hollow molded plastic product is removed.
Numerous rotational molding apparatus and processes are known for molding hollow plastic containers. Such are illustrated, for example, in U.S. Pat. No. 4,167,382 to Freedman et al., U.S. Pat. No. 4,722,678 to Wersosky, U.S. Pat. No. 4,836,963 to Gilman, Jr., U.S. Pat. No. 3,829,272 to Carillon et al., U.S. Pat. No. 3,907,482 to Shiota et al., U.S. Pat. No. 4,285,903 to Lemelson and U.S. Pat. No. 4,755,333 to Gray. All of these methods and apparatus are adapted for the rotational molding of hollow plastic forms. However, prior art rotational molding apparatus have heretofore been substantially limited in the number of molds that could be handled at one time.
In addition, it has not heretofore been appreciated to provide a rotational lining apparatus and method whereby an enclosed structure such as a water tank or storage drum or the like can be lined with a polymeric material using a rotational application technique. In the prior art, structures such as water tanks and the like have traditionally been lined with glass linings or metal linings, or have been utilized unlined. Many prior art methods and apparatus have been utilized for the purpose of lining structures such as water heaters and the like to prevent corrosion and extend their useful life. For example, U.S. Pat. No. 3,275,798 discloses a domestic water heater with the inside of the tank coated with a layer of cellular plastic foam 28 and discloses several methods of applying such foam including spraying on the inner wall surface and centrifugally casting the foam on the inner wall surfaces. It is recognized in U.S. Pat. No. 3,275,798 that when lining such a tank by centrifugally casting the foam on the inner wall surfaces, the end walls require special handling to coat. The process described with regard to centrifugal casting is to rotate the tank slowly while foaming ingredients are being distributed over the inner wall surfaces of the tank, and then to increase the rotation rate to evenly distribute the mixture along the wall surfaces of the tank. This rotation fails to properly coat the end surfaces of the tank and requires additional handling and processes to completely line a tank structure. Another example is provided in U.S. Pat. No. 4,783,896 which discloses a method of making a cathodically protected water heater. This patent discloses lining a water heater with a coating of metal that is anodic to steel, such as aluminum, magnesium, zinc or alloys thereof. The '896 patent also discloses and discusses glass lining of water heaters and the problems thereof.